Due to warming oceans, precipitation in the form of rain and snow fall will increase as more water vapor enters the atmosphere. Precipitation is also expected to increase over the 21st century, particularly at northern mid-high latitudes, though the trends may be more variable in the tropics, with much of the increase coming in more frequent heavy rainfall events. However, over mid-continental areas summer-drying is expected due to increased evaporation with increased temperatures, resulting in an increased tendency for drought in those regions. Source: http://lwf.ncdc.noaa.gov/oa/climate/globalwarming.html